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From “The Body Keeps the Score” by Bessel van der Kolk, M.D.,
“The emotional brain has first dibs on interpreting incoming information.
Sensory Information about the environment and body state received by the eyes, ears, touch, kinesthetic sense, etc., converges on the thalamus, where it is processed, and then passed on to the amygdala to interpret its emotional significance.
This occurs with lightning speed.
If a threat is detected the amygdala sends messages to the hypothalamus to secrete stress hormones to defend against that threat.
The neuroscientist Joseph LeDoux calls this “the low road.”
The second neural pathway, the high road, runs from the thalamus, via the hippocampus and anterior cingulate, to the prefrontal cortex, the rational brain, for a conscious and much more refined interpretation.
This takes several microseconds longer.
If the interpretation of threat by the amygdala is too intense, and/or the filtering system from the higher areas of the brain are too weak, as often happens in PTSD, people lose control over automatic emergency responses, like prolonged startle or aggressive outbursts.”
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PTSD - A Way Out.com 